Archelon
Archelon is an extinct marine turtle from the Late Cretaceous, and is the largest turtle ever to have been documented, with the biggest specimen measuring 460 cm (15 ft) from head to tail, 400 cm (13 ft) from flipper to flipper, and 2,200 kg (4,900 lb) in weight. It is known only from the Dakota Pierre Shale and has one species, A. ischyros. In the past, the genus also contained A. marshii and A. copei, though these have been reassigned to Protostega and Microstega respectively. The genus was named in 1895 by American paleontologist George Reber Wieland based on a skeleton from South Dakota, who placed it into the extinct family Protostegidae. The leatherback sea turtle (Dermochelys coriacea) was once thought to be its closest living relative, but now, Protostegidae is thought to be a completely separate lineage from any living sea turtle. Archelon had a leathery carapace instead of the hard shell seen in sea turtles. The carapace may have featured a row of small ridges each peaking at 2.5 or 5 cm (1 or 2 in) in height. It had an especially hooked beak and it jaws were adept at crushing, so its diet probably consisted of hard-shelled crustaceans and mollusks while slowly moving over the seafloor. However, its beak may have been adapted for shearing flesh, and Archelon was likely able to produce powerful strokes necessary for open-ocean travel. It inhabited the northern Western Interior Seaway, a mild to cool area dominated by plesiosaurs, hesperornithiform seabirds, and mosasaurs. It may have gone extinct due to the shrinking of the seaway, increased egg and hatchling predation, and cooling climate. Taxonomy Research history The holotype specimen, YPM 3000, was collected from the Late Campanian-age Pierre Shale of South Dakota along the Cheyenne River in Custer County by American paleontologist George Reber Wieland in 1895, and described by him the following year based on a mostly complete skeleton excluding the skull. He named it Archelon ischyros,2 genus name from the Ancient Greek ἀρχη- arkhe- "first/early,"3 χελώνη chelone "turtle,"4 and species name from ἰσχυρός ischyros "mighty" or "powerful".5 Wieland placed it into the family Protostegidae, which included at the time the smaller Protostega and Protosphargis.2 The latter is now in the family Cheloniidae.6 A second specimen, a skull, was discovered in 1897 in the same region.7 In 1900, Wieland described a second species, A. marshii, from remains collected in 1898 by American paleontologist Othniel Charles Marsh, to whom the species name refers, on the basis that the shell underside (plastron) was thicker and the humeri were straighter.7 However, in 1909, Wieland reclassified it as Protostega marshii. In 1902, a third, mostly complete specimen was collected also along the Cheyenne River.8 In 1953, Swiss paleontologist Rainer Zangerl split Protostegidae into two families: Chelospharginae and Protosteginae; to the former was assigned Chelosphargis and Calcarichelys, and the latter Archelon and Protostega.9 In the same study, the Kansas Protostega copei, which was first described by Wieland in 1909 and named in honor of Edward Drinker Cope who first erected the family Protostegidae,8 was moved to the genus Archelon as A. copei.9 In 1998, A. copei was moved to the new genus Microstega as M. copei.10 In 1992, a fourth and the largest specimen to date, nicknamed "Brigitta," was discovered in Oglala Lakota County, South Dakota and resides in the Natural History Museum Vienna.11 In 2002, a fifth specimen, a partial skeleton, was discovered from the Pierre Shale of North Dakota along the Sheyenne River near Cooperstown. Evolution The sister group of Protostegidae has, in the past, been considered to be Dermochelyidae, and thus their closest living relative would have been the dermochelyid leatherback sea turtle (Dermochelys coriacea).10 However, phylogenetic studies conclude that protostegids represent a completely separate, ancient (basal) lineage that originated in the Late Jurassic, removing the family from the superfamily Chelonioidea which includes all sea turtles. In this model, Archelon does not share a marine ancestor with any sea turtle, neither modern nor contemporary. |cladogram= |2= }} |4= }}}}}}}}}}}} Description The holotype measures 352 cm (11.5 ft) from head to tail, with the head measuring 60 cm (2 ft), the neck 72 cm (2.4 ft), the thoracic vertebrae 135 cm (4.4 ft), the sacrum 15 cm (0.5 ft), and the tail 70 cm (2.3 ft).2 The largest specimen, Brigitta, measures around 460 cm (15 ft) from head to tail and 400 cm (13 ft) from flipper to flipper,1211 and, in life, weighed around 2,200 kg (4,900 lb).17 Archelon had a distinctly elongated and narrow head. It had a defined hooked beak which was probably covered in a sheath in life, reminiscent of the beaks of birds of prey. However, in the back, the cutting edge of the beak is dull compared to sea turtles. Much of the length of the head derives from the elongated premaxillae–which is the front part of the beak in this animal–and maxillae. The jugal bones, the cheek bones, due to the elongate head, do not project as far as they do in other turtles. The nostrils are elongated and rest on the top of the skull, slightly posited forward, and are unusually horizontal compared to sea turtles. The jugal bones (cheekbones) are rounded as opposed to triangular in sea turtles. The articular bone, which formed the jaw joint, was probably heavily encased in cartilage. The jaw probably moved in a hammering motion. Five neck vertebrae were recovered from the holotype, and it probably had eight in total in life; they are X-shaped, procoelous–concave on the side towards the head and convex on the other–and their thick frame indicates strong neck muscles. Ten thoracic vertebrae were found, increasing in size until the sixth then rapidly decreasing, and they have little connection with the carapace. The three vertebrae of the sacrum are short and flat. It probably had eighteen tail vertebrae; the first eight to ten (probably in the same area as the carapace) had neural arches, whereas the remaining did not.2 Its tail likely had a wide range of mobility, and the tail is thought to have been able to bend at nearly a 90° angle horizontally.18 The humeri in the upper arms are proportionally massive, and the radii and ulnae of the forearms are short and compact, indicating the animal had strong flippers in life. The flippers would have had a spread of between 490 and 610 cm (16 and 20 ft), though most likely the more conservative estimate.19 Stretch marks on the limb bones indicate fast growth,20 with similarities to the leatherback sea turtle, the fastest growing turtle known,21 whose juveniles have an average growth rate of 8.5 cm (3.3 in) per year. Carapace The carapace comprises on either side eight neuralia–the plates closest to the midline–and nine pleuralia–the plates that connect the midline to the ribs. The plates of the carapace are mostly uniform in dimensions, with the exception of the two pairs of plates corresponding to the eighth thoracic vertebra which are smaller than the others, and the pygal plate closest to the tail which is larger. Archelon has ten pairs of ribs, and, like the leatherback sea turtle but unlike other sea turtles, the first rib does not meet the first pleural. As in sea turtles, the first rib is noticeably shorter than the second, in this case, three quarters of the length. The second to fifth ribs project at a right angle from the midline, and, in the holotype, each measure 100 cm (3.3 ft) in length. A rib increases in thickness in the vertical direction distally, as it gets farther from the midline, and the ribs are relatively larger and more well-developed than those of sea turtles. The second to fifth ribs, in the holotype, originate with a thickness of 2.5 cm (0.98 in) and terminate with around 4 to 5 cm (1.6 to 2.0 in) in thickness. The neuralia and pleuralia form highly irregular and finger-like sutures where they meet, and one plate may have lain over the other plate while the bone was still developing and malleable. The neuralia and pleuralia–the bony portions of the carapace–are particularly thin, and the ribs, especially the first rib, and the shoulder girdle are unusually heavy and may have had to carry extra stress to compensate, a condition seen in ancient ancestral turtles.198 Archelon has osteosclerotic structures, where the bone is dense and heavy, which probably served as ballasts in life similar to the limb bones of whales and other open-ocean animals.22 The carapace, in life, probably featured a row of ridges along the midline over the chest region, perhaps totaling in seven ridges, with each ridge peaking at either 2.5 or 5 cm (1 or 2 in).18 In the absence of firmly jointed neck and pleural plates, the skin over the carapace was probably thick, strong, and leathery in order to compensate and properly support the shoulder girdle.19 This leathery carapace is also seen in the leatherback sea turtle. The spongy makeup is similar to the bones seen in open-ocean going vertebrates such as dolphins or ichthyosaurs, and was probably also an adaptation to reduce overall weight. Plastron A turtle plastron, the underside, comprises, from head-most to tail-most, the epiplastron, the entoplastron, which is small and wedged in between the former and the hyoplastron, then, following, the hypoplastron, and finally the xiphiplastron. The plastron, as a whole, is thick,18 and measures, in a specimen described in 1898, 210 cm (7 ft) in length.8 Unlike the carapace, it features striations throughout.23 In protostegids, the epiplastron and entoplastron are fused together, forming a single unit called an "entepiplastron" or a "paraplastron." This entepiplastron is T-shaped, as opposed to the Y-shaped entoplastrons in other turtles. The top edge of the T rounds off except at the center which features a small projection. The outward side is slightly convex and bends somewhat away from the body. The two ends of the T flatten out, getting broader and thinner as they get farther from the center.23 A thick, continuous ridge connect the hyoplastron, hypoplastron, and xiphiplastron. The hyoplastron features a large number of spines projecting around the circumference. The hyoplastron is slightly elliptical, and grows thinner as it gets farther from the center before the spines erupt. The spines grow thick and narrow towards their middle portion. The 7 to 9 spines projecting towards the head are short and triangular. The 6 middle spines are long and thin. The last 19 spines are flat. There are no marks indicating contact with the entepiplastron. The hypoplastron is similar to the hyoplastron, except it has more spines, a total of 54.23 The xiphiplastron is boomerang-shaped, a primitive characteristic in contrast to the straight ones seen in more modern turtles. Paleobiology Category:Cretaceous turtles Category:Prehistoric reptiles of North America Category:Sea turtles